1. Field of the Invention
The present invention relates to a CMOS logic circuit, an AD converter and DA converter using capacitive coupling, and to an innovative circuit that can reduce the number of circuit elements required for logic circuits, AD converters and DA converters, which together with a photoelectric conversion device, such as an image sensor, are integrated in a single LSI.
2. Related Arts
For the fundamental structure of a CMOS logic circuit, comparatively many CMOS transistor elements are employed, as, for example, is described in "Base of MOS Integrated Circuit" (Kindai Kagakusha, May 30, 1992). AD converters are described, for example, in "Transistor Techniques Special, No. 16" (CQ Publication, Feb. 1, 1991, Second Edition). A flash AD converter that can obtain output at the same time as input being applied is better as an AD converter integrated with a photoelectric conversion device, such as an image sensor. However, the flash AD converter requires many comparators, and accordingly, the volume of the circuit is increased. For example, 2.sub.n -1 comparators are required to constitute an n-bit AD converter.
An image sensor has been studied wherein a signal processing function is provided on the chip by forming a CMOS circuit on the same substrate and whereby digital output is enabled. Such an image sensor is described, for example, in "Proceedings of SPIE Vol. 2745 Infrared Readout Electronics III" pp. 90-127, Apr. 9, 1996.
As is described above, however, the current CMOS logic circuit has many elements, and the AD converter which converts an analog signal detected by a sensor into a digital signal also has many elements. If a digital circuit employing these circuits is formed on the same substrate as is a photosensor, a fill-factor, which is a ratio of the sensor area to the area of the entire chip, is extremely small. This is described, for example, in ISSCC 1994 Digest Of Technical Papers, pp. 230.